The present invention relates to a separator of a battery, the separator comprising a porous substrate and a heat-resistant porous layer, wherein the heat-resistant porous layer comprises a 2-cyanoethyl group-containing polymer having a specific molecular weight distribution and a weight-average molecular weight as a binder; and a non-aqueous electrolyte battery with improved heat resistance comprising the separator.
Recently, as a power source for a mobile terminal such as that of a notebook computer or a cellular phone or as a power source for a hybrid car or an electric car, a non-aqueous electrolyte battery with high voltage and high energy density, in particular a lithium ion secondary battery, is getting the attention.
The non-aqueous electrolyte battery typified by a lithium ion secondary battery has high capacity and high energy density so that a large electric current flows at the time of internal short circuit or external short circuit of the battery. Thus, there is a problem that heat is generated in the battery due to Joule heat caused by short circuit, the battery is swelled due to gas generation accompanied with decomposition of an electrolyte solution, and properties of the battery are deteriorated.
According to a current lithium ion secondary battery, in order to resolve such a problem, a separator comprising a porous substrate having fine pores such as a polypropylene or polyethylene film is interposed between a negative electrode and a positive electrode. When the temperature increases owing to the heat generated by short circuit, the separator comprising the porous substrate melts to block the pores. As a result, movement of ions is inhibited so that the current does not flow and runaway of the battery is suppressed.
Because of wider use of a lithium ion secondary battery, a battery having higher heat resistance, in particular improved heat resistance at the time of internal short circuit has been currently required.
When the internal short circuit occurs, it is believed that the temperature increases to 600° C. or higher at the short circuit region owing to local heat generation. Thus, in a conventional separator comprising a porous substrate having fine pores such as a polyolefin film, the separator is shrunken or melted by heat generated by the short circuit at the short circuit region so that the battery is exposed to dangers of fuming, ignition and explosion.
As a method for preventing the short circuit caused by heat shrinkage or heat melting of a separator and improving reliability of a battery, a multilayer separator comprising a heat-resistant porous layer on one or both surfaces (i.e., frond and back surfaces) of a porous substrate having fine pores such as a polyethylene film is suggested (JP 2010-015917A; JP 2009-518809T, which is a National Phase Publication of WO 2007/066967; JP 2008-503049T, which a National Phase Publication of WO 2006/004366; and JA 2010-50076A). It is also disclosed that a 2-cyanoethyl group-containing polymer is used as a binder in a heat-resistant porous layer (JP 2009-518809T and JP 2008-503049T).